Decision point or speed?

[Spit Murphy]

G'day all,

The mob I work for fly out of very short strips. During the take off brief there are always different ideas on using a decision point or speed. The op's manual is a bit grey in this area.

Do you use a Decision point, speed or both ie.

We will continue the take off once passed the windsock and above red line. If we have a failure below one of those points we abort the take off.

Thanks for your help.

[404 Titan]

Spit Murphy

It’s been a while since I flew a piston twin but I use to use a decision point on long strips and a decision speed from short strips. The logic in this was that if I had runway in front of me and I could safely land back down on it, it was safer to land the thing than take the problem into the air where it could possibly get ugly.

[QSK?]

I was given some very good and simple advice by an old pilot friend years ago for getting out of short, or performance degrading, strips, viz:

If you havn't achieved three-quarters of your calculated TOSS by the runway half-way point, then abort!

This advice was complementary to consulting the aicraft P-charts.

I have found this rule of thumb to be quite workable over the years, but have never really tested it's validity or integrity against the advice of others.

So, I'd also be interested in any comments on this method.

[turbantime]

QSK?
Just curious to how you worked out that this advice was complementary to P-charts.

404 Titan
What kind of decision points/speeds did you use?

Curious as I haven't had to land bigger aircraft into smaller strip but I'm sure the day will come soon :ooh:

Would also like to know what kind of decision points/speeds are used by pilots of singles too.

Cheers :ok:

[ROB-x38]

Very interesting topic this. As a freshly rated light twin driver I have thought about this quite a bit.

I was taught a decision speed. Where i'm based there is plenty of runway, so our decision speed is increased to Vyse (ie: after rotation) so if an engine quits before this speed but after rotation, procedure is to close the throttles and land on remaining runway available. Although a decision point would work just as well i reckon, and maybe even more appropriate for a longer strip.

Out of shorter strips you don't have that luxury, and i'd use a speed, the max decision speed being rotate speed. Although my multi-engine command column is in single figures so i'm no authority :O
Would also like to know what kind of decision points/speeds are used by pilots of singles too.
Unless i'm missing something what decision?

[Bevan666]

One thing that just using a decision speed is that it does not take into account acceleration. If you're not accelerating normally (due to altitude, surface condition, winds or slope) waiting until you have a specific speed may see you off the end of the strip. Using both a decision speed (Go/No go) for an engine failure and a decision point (Go/No Go) for acceleration is the best option.

Now I was taught and have used the 3/4 Vtoss by the halfway point on the strip as a great guide to whether you are accelerating at a rate to achieve the required takeoff performance. This of course, is applicable to any aircraft, regardless of the number of engines.

If you want to know more about decision points etc, I recommend reading the F.E. Potts guide to Bush Flying. See this link (http://www.fepco.com/Bush_Flying.html)
. This book covers a lot of issues about operating from small strips which I havent seen covered elsewhere.

[speedjet]

Ever thought of reading the Aircraft Flight/Operating Manual.

I know the C402 used to have in Emergency Procedures - Engine Failure on Take-off below 95 knots - ABORT, above 95 knots - CONTINUE.

So my opinion use a speed not a decision point.

Recently shown video where statistics indicate that pilots that try to re-land on remaining runway after engine failure above V1 are worse off than those that elect to continue and do a circuit and re-land.

[Blown Seal]

Spit Murphy,

It may help a little if you provide some clarification on what your definition of a short strip is as most of the other posts have all mentioned long and shorts strips? Where you'retalking about might come in handy too.

Some short strips (550m and less), as in the Torres Straits, IMO deserve to have a decision point rather than a speed. This is based on being able to stop on the remaining rwy if a failure occurs prior to the chosen point, or having a failure past (but<Vdec/yse) that point then getting airborne, being able to lower the nose, and gain all valuable airspeed and hopefully keep control. This is in preference to aborting a take-off post rotation and conducting a ditching or forced landing.

Obviously there are locations where you do not have the luxury of being able to descend (or land) after the end of the rwy, in this instance I too would use Vr because I'd prefer to have a fighting go at saving the a/c than just chopping the power and putting it into the trees.

Hope this helps a little.

Blown.:ok:

[The Bullwinkle]

Blown Seal

I would definitely agree with you there.

When taking off from Darnley Island (approx 400m), I used to choose a decision point, beyond which, I would commit to the take-off, even if the failure occured before Vr.

The reason for this was that there is a rather steep drop off at the end of the strip, so rather than aborting and not coming to a complete stop on the runway, I would prefer to use the 200 foot drop-off at the end of the strip to accelerate the aircraft, and then if not climbing, a controlled descent in a level attitude onto the beach or the water's edge.

Anyway, that's just what I used to do.

BW

[Not_Another_Pot]

I seem to remember a 50/40 rule.... 50% of airspeed by 40% of the strip then continue if not stand on the brakes!!!!!

NAP

[turbantime]

ROB-x38,
I realise that this thread is more in tune with twin ops but what I would like to know is whether the 3/4 TOSS by half strip etc would also work in a single.

There'd have to be a point where the A/C would just not reach desired performance for take-off and "the decision" would be to abort the take-off.

Have known of a couple of singles to go off the end while on a "comfortable" strip and would like to know of a proven method so I can save myself from being a stat one day :ugh:

By "proven" I mean, who did and how were the numbers crunched from P-charts.....if at all.

[Spit Murphy]

To Bull and Blown Seal,

I might have read your post wrong but what happens if you take off from Darnley or Mabuiag and have an engine failure past your decision point but below Vmca. You can't stop before the end of the runway and if you decide to keep going you will not have any rudder control. That is why I tend to use a decision point with speed also. I might be missing something here and that is why I asked for advice.

After re-reading your post I think you guys sort of do the same as what I am doing. I think I would keep it straight as best I could and fly it of the strip in ground effect.

Thanks for the advice everyone.

[The Bullwinkle]

It has been a while, but from memory, the VMCA for an Islander was only 39 knots, and this was generally achieved well before the decision point. (I also preferred to have full power before releasing the brakes in these situations)

I always felt that a rejected take-off which resulted in an over-run at low speed, would almost certainly be fatal at Darnley, whereas continuing would hopefully be more controllable and therefore survivable.

I am just glad that I never had to put this theory into practice!

BW

[QSK?]

Turbantime:

I realise that this thread is more in tune with twin ops but what I would like to know is whether the 3/4 TOSS by half strip etc would also work in a single.
I fly a single and I have found from my experience that the 3/4 Vtoss by the runway mid-point works very effectively; and Bevan 66's post would appear to confirm this. The 50% Vtoss at the 40% runway point as mentioned by Not_Another_Pot is a new one to me, and one that I haven't tried. Somewhere in my collection is a magazine article on the "science" behind the 75/50 rule. If I find it, I'll PM you.

Wrt to your query on my eariler post, probably bad wording on my part. All I was trying to say is that, firstly, the pilot must still consult the P-Charts on the day to determine TODR and the relevant Vtoss, then apply the 3/4 TOSS by 50% distance rule of thumb.

[Bevan666]

To prove the 75% Vtoss by 1/2 strip length requires a bit of mathematics.

Assuming constant acceeration, at the 1/2 way point in your takeoff roll;

V1 = SQRT(V2^2/ 2)

where V1 = 1/2 way speed and V2 is Vtoss.

(Using newtons second law of motion).

Putting in some numbers, with a Vtoss of 75 knots;

V1 = 53 knots.

Now this is awfully close to 3/4 of Vtoss which is 56 knots. Now most of us cannot do this sort of maths in our head, so this rule of thumb gives reasonably accurate answers. So if you pass mid way on a strip with less than 56 knots, abort or you'll leave the end. If you pass mid way with more than that figure, you have room to spare - go.

Being in a single or twin will not make any difference. If a donk lets go after the mid point, your acceleration will reduce (or stop completely if you are in a single) and depending on length you will end up off the end, or may have enough to climb away.

This rule of thumb is good enough for me.

Bevan..

[dogcharlietree]

Decision point or speed?
It's a good idea to use both.
Unless you are flying a Learjet, there is a couple of seconds between those speeds.
You check your (precalculated) 50% runway length speed prior to rolling, then if this is exceeded, continue to your V1 speed, etc.
It's not hard and gives a very good indication as to how your aircraft is performing reasonably early in the take-off run.
Rule of Thumb #12 in The Axioms of Flight states, "Abort the takeoff if 70% of takeoff velocity is not attained within 50% of available runway"
Personally, I think the figure of 75% is far easier to mentally calculate and will also put you on the safer side.

[Desert Duck]

Decision point - based on my assessment of speed, height and rate of climb will be where I decide that the land ahead option is no longer available.

Single engine speed is a factor - if everthing goes my way we may continue to fly or
controlled or uncontrolled flight into terrain

Marginal length strips are a serious compromise.

[404 Titan]

turbantime

Not sure how those that fly out of short strips can use a decision point. In any piston twin on a short strip, your decision to go or not go is going to be made base on whether you have attained Vyse or not, i.e. will the aircraft perform? By the time you have attained Vyse at a place like Darnley or Mabuiag you will have insufficient runway to land on straight ahead. You are going to need more than 100 ft if you are going to trade height for speed if you elect to go before Vyse. Making a decision to go before you have reached Vyse is fraught with danger. In all piston engine twins there is a grey area between Vtoss and Vyse where if something goes wrong with an engine, the aircraft will not perform assuming ISA conditions and an aircraft at MTOW. Unfortunately all flight manuals that I have seen only give one figure for Vyse, so it makes it difficult to determine what the real Vyse is if you are lighter than MTOW or colder than ISA. Conditions much hotter than ISA in many piston twins have shown that the aircraft won’t perform on one engine, period. You are going to have to use the good engine to make a forced landing.

As for using a decision point. I only ever used it out of Cairns or Townsville, as the strips were very long. The decision point must always be the same point over the ground as Vyse or beyond it and there must be sufficient runway to land straight ahead. If you can’t meet these requirements then you are going to have to use Vyse, i.e. a decision speed and accept that if an engine fails before this speed you are going to have to make a forced landing with the good engine taking you there.

[turbantime]

QSK?
Thanks for the clear up, should have understood that that's what you meant to say :8

Bevan666,
Those numbers work quite well, I'm the kind of person that likes proof before trying rules of thumbs out (it instills more confidence in me) :D

404 Titan,
After thinking about the topic, I am too in line with thinking that twin ops would need a decision speed and that speed would be Vyse......and the rest would be left to as "on the day decision" which'd be covered in the take-off brief...ie more than sufficient runway, so land back on it.....or the 200 ft drop off which would allow further acceleration etc

Cheers for taking the time guys :ok:

[D Rate]

Great Topic.

NAP has it right...
It really is a combination of both. In the operation of jet aircraft as in the piston twin it is a speed...BUT WHAT WE ARE LOOKING AT WHEN WE CONSULT THE GRAPHS IS A SPEED WHICH ALLOWS US SUFFICIENT DIRT/BITUMEN IN METRES TO STOP!!

If you think of it as having the brakes stuck on, you might get to the desired speed when well into the trees. So the best approach is to remember that the speed can be deceptive and use a combination of speed and a distance, be it 40% of the field length or such and if you havent achieved that speed or the acceleration required, jump on the brakes!!!

Be safe!

[404 Titan]

Having a decision point on the ground is OK for acceleration determination purposes, but is useless as a go, no go point if you have an engine failure as the original question was alluding to in this post. If you haven’t reached Vyse and you decide to continue, you will crash or worse be killed. All final decisions to continue or not must be referenced to Vyse. If you are taking off from a long strip, sure use a decision point, but reference it to where Vyse will occur over the ground. It MUST occur at that point or beyond. If it doesn’t you are committing yourself to a take off that the aircraft CAN NOT recover from and you and your passenger may die. Any talk about jet aircraft is irrelevant to this discussion as they are designed to be able, at V1 to abort the take-off and stop on the remaining runway OR continue the take-off and fly away. Piston twins aren’t, and there the differences lie and people must accept these differences because they are fundamental in how both types of aircraft will be operated at take-off and in other phases of flight in emergencies.

[Blown Seal]

Titan,

If you haven’t reached Vyse and you decide to continue, you will crash or worse be killed.

Isn't this a little extreme? Do you mean Vmca?

[404 Titan]

Blown Seal

No I don’t. It goes without question that if you try and continue a takeoff below Vmca with an engine out you will most definitely crash and die. If you press on with a take-off below Vyse, the aircraft will not perform and the other engine will take you to the crash. If though you realize beforehand and brief accordingly that the aircraft won’t perform, you should have a plan of action about where you will do a forced landing using the good engine to get you there if required. If you don’t do this you may try and extract a climb performance out of the aircraft that it can’t give you, thereby possibly taking the aircraft below Vmca and the resultant loss of control.

I have been deliberately blunt in what I have said because it would appear that some that have posted here don’t realize these fundamental dangers in piston twin operations, particularly at take-off. :sad:

[The Bullwinkle]

404 Titan

I understand what you are saying and I realise that continuing a take-off in a piston twin on one engine below Vyse is not healthy. However, as in the Darnley example where there is a 200 ft drop, if you reach a point where the aircraft cannot be stopped before the end of the runway after an engine failure, you will most certainly die or be severely injured.

I would still prefer to run off the end of the strip at darnley, and use the good engine to carry me in a level attitude to the crash site, rather than plummet over the edge with absolutely no control whatsoever.

[404 Titan]

The Bullwinkle

In reality it sounds like you have made a decision to abort the take-off and use the good engine to take you to the forced landing sight. I have no problem with that. I am very aware of the dangers of Darnley as I use to fly to the Straits all the time when I worked in Cairns. The term decision point or speed refers to the ability to continue the take-off if an engine should fail based on the aircrafts performance. Having a point on the runway to confirm that the aircraft is accelerating normally is great and we should all have one, but it isn’t the decision point or speed that can be used in the event of an engine failure.

[5 Left & Right]

Haven't seen anyone mention Vtoss here yet!

A point for a long strip and speed for a short strip is in most cases correct except perhaps for extremes like Darnley or Murray Is. in the straits (500-600mtrs)

But what is short and what is long?

Operating twin cessnas around the traps i use a ball park 1000m. That is if its less than 1000m i'll make the decision a speed, if its more than 1000m then i'll use a point (usually gear up ie. that point where u can no longer land on the remaining rwy) and by that time you will always have a safe speed to go.

Dont know about 75% of this by 50% of that, sounds like gobbledy gook to me, never was good at maths. If you're that worried about it dig out a P chart and draw a line thru it.

The speed (if thats the case ie short strip) should be, ideally, sufficient to allow you to fly away. Vyse theoretically will do that for you, and 404 titan, it is published in the POH for different weights for good machines like cessnas! (your memory must be fading buddy!)

If you choose to GO with less than VYSE or at VYSE with the gear or flap still hanging out you will have to be prepared to sacrifice height to gain speed (option at Darnley or Murray with 200-300ft elevation) as the ga twin will not be able to accelerate to Vyse one engine out and maintain level flight.

The question always comes around that "well what if i was at Vyse minus 5kt?" then it really comes back to weight i think. If you are at MTOW then you must have height to sacrifice, if you are light then you may just pull it off.

When light out of a short strip i would be prepared to give it a go at anything above Vtoss presuming the gear is on the way up, Vtoss is a guarantee of control over the stall & Vyse, it does not imply performance but at light weight i reckon it would go ok. If not then it would set you up nicely for a ditching or a controlled crash.

To the bloke who started this post - good onya, good question, but no simple answer, lots & lots of variables

Cheers ;)

[404 Titan]

5 Left & Right

Hey good to hear from you. How’s Sunny FNQ treating you? I did mention Vtoss earlier. I stated that there was a huge gray area between Vtoss and Vyse. As you and I know, Vtoss should be the greater of 1.2 x Vs (stall speed clean) or 1.1 x Vmca. In a lot of aircraft there can be a large gap between Vtoss and Vyse like the PA60. In some it is small like the BN2. My memory of the C404 and all the other Cessna twins you and I operated in FNQ is fast fading from the gray matter as I have many many more numbers running around my head these days, so I will take your word for the C404 POH. After all you have a lot more hours on the machine now than I do.

In regards to your decision speed, I also agree. What is vital is that pilots must brief what they will do if they don’t get the performance they were expecting or the failure occurs at a critical part of the take-off, i.e. before Vyse. They must have the discipline to follow through with their actions even though it will result in a forced landing and the possibility of getting hurt.
:ok:

[piontyendforward]

Good discussion, PPrune at its best.

The 75/50 rule of thumb is not a decision point for an E.F. It is a decision point for runway length and as an acceleration check. As such it will work for all aircraft. The P charts, AFM, etc are a static calculation, usually done before take off, the 75/50 rule of thumb is a rolling "how goes it", that will cover most unplanable type events.

It is great for those days when there are some puddles on the runway, or the grass is a bit longer that usual, or a tail wind increases during takeoff run etc.

Most jet pilots I know do a similar check, but use 80 knots at the 1000 foot blocks as a rough guide to how the machine is accelerating.

It does not tell anything about engine out performance and capability, so cannot be used as a M.E. decision point for the GO/NOGO decision for engine out.

The stay on the ground or go flying decision for multi engine aircraft with an E.F. is only an airspeed, weather you are actually going to be able to make that speed (with all engines running) in the distance available is where the 75/50 rule of thumb comes into play.

[Blown Seal]

From the "authorities":

http://www.caa.co.uk/docs/33/SRG_GAD_SSL07.PDF

Section 6 on page 5 puts it pretty succinctly.

Titan - whilst I do not wish to enter into an argument over this because it could keep going round in circles - WRT my previous post I was merely questioning your statement (generalization?), in some circumstances it would be correct but certainly not for all situations.;)

[404 Titan]

Blown Seal

There is nothing in the article that I haven’t already said and agree with. Please read carefully again what I have written. My point was that the so called decision point on the ground is misleading because it isn’t the final decision point or speed the pilot will have to make. The final one will be in the air. The article talks about twin engine aircraft not achieving the scheduled single engine rate of climb until clean and blue line. It also says that under limiting conditions ( of which I have stated a few), there may be no rate of climb at all. Have a look in the plane you are flying now for its single engine ceiling for different weights. Not very impressive and that is in ISA. Now do a calculation of the average density altitude that you take-off from. Even at light weights you may not perform. In the vast majority of Australia this is going to be a problem in the Summer and virtually always in the north.

QUOTE:
____________________________________________________
Decision point : you should work out the runway point at which you can stop the aeroplane in the event of engine or other malfunctions e.g. low engine rpm, loss of ASI, lack of acceleration or dragging brakes. Do NOT mentally
programme yourself in a GO-mode to the exclusion of all else. If the ground is soft or the grass is long and the aeroplane is still on the ground and not accelerating, stick to your decision-point and abandon take off. If the grass is wet or damp, particularly if it is very short, you will need a lot more space to stop.

Twin engines : on twin engined aircraft, if there is an engine failure after lift off, you may not reach the scheduled single engine rate of climb until:
• the landing gear and flaps have retracted (there may be a temporary degradation as the gear doors open)
• the best single engine climb speed, ‘blue line speed’, has been achieved.

Under limiting conditions (translation: MTOW, Hotter and/or lower pressure than ISA and/or elevation higher than MSL) an engine failure shortly after lift off may preclude continued flight and a forced landing will be necessary. Where the performance is marginal, the following points must be considered when deciding the best course of action:
• while flying with asymmetric power it is vital that airspeed is maintained comfortably above the minimum control speed, VMC. A forced landing under control is infinitely preferable to the loss of directional control with
the aircraft rolling inverted at low altitude. If there are signs you are losing directional control, lower the nose immediately if height permits to regain speed and if all else fails reduce power on the operating engine. (Care
must be taken to maintain normal margins above the stall.)
• performance and stall speed margins will be reduced in turns. All manoeuvres must be kept to gentle turns.
____________________________________________________